The aim of this study is to determine the structure of enamel and the ultrastructure of the cells involved in amelogenesis in several mammals using both scanning and transmission electron microscopes. The goal is to enhance our understanding of amelogenesis and enamel structure and to provide a basis for evaluating animal model systems by comparing rat, cat, monkey, and human teeth. Differences in enamel structure and formation among these mammals will be noted, particularly variations from that observed in humans. Emphasis will be given to uncovering basic principals of amelogenesis applicable to all enamel systems and possible differences between rapidly and slowly forming teeth. The studies will be directed to enamel formation at the tooth surface of several mammals and at the fissures of rat molar teeth. Phenomena pertaining to the termination of enamel rod elaboration, surface aprismatic enamel deposition, and the formation of lines of Retzius will be examined. In this investigation of amelogenesis we will use standard techniques for transmission and scanning electron microscopy. Additionally, for scanning microscopy, we will use our approach of separating ameloblasts so that the lateral surfaces of the cells and the enamel-ameloblast interface can be examined.